The bandwidth of a digital signal depends on the data rate. Doing real time streaming digital voice requires far more bandwidth than sending characters at a typing rate as in PSK31 or even RTTY. The result is that there is no great bandwidth benefit to using real time digital voice over analog SSB.

There is currently some digital voice being used on HF. The biggest issue is that you must be tuned to the exact frequency at the beginning of a transmission in order to properly synchronize and decode the digital voice signal. The result is that most people make the initial contact with SSB and then switch to digital voice. Its fun to experiment with but it in its current state it doesn't seem to be very practical for general HF amateur use unless we eliminate the VFO and go to a channelized arrangement.

I think the answer to this question is not so much technical than sociological.It is the same reason that you will find many more people on PSK31 than Olivia for example, or that many more people use PC platforms than Macs.Once a technology gets a certain critical mass of users, success builds on success and the greater number of users will attract even more users, resulting in a snowball effect.Although I love Olivia, and am continually astounded at how well it decodes weak signals, I find it difficult to consistently get QSO's on this mode. Granted, there is a small group of enthusiasts, but for consistent QSO ability, I find myself heading back to PSK31.Digital voice is in the same category as exotic digital modes - the technology works, but suffers from lack of a consistent user base.It is no more difficult to run most exotic digital modes than ticking a box on the PC screen, so it is not the technology which is the problem. Having to arrange qso's or join a net to try to qso is an interesting transient exercise, but will restrict the number of participants even further.In my experience, ham radio is definitely stratified into layers of users who tend to remain with certain modes.A large number of my SSB voice contacts have never tried digital modes, nor are they particularly interested in them. And many digimodes operators have forgotten where they stored their microphone! This is not not a criticism, but just a reflection of an activity which is after all a hobby, and as such, should cater to the particular individuals interests.Digital voice is in the situation of falling between SSB operators who dont see the advantage of the extra complexity, and digital operators who are more interested in typing than talking.Vive le difference!

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